Clipboard Not Working on Ubuntu 18.04 in Blender 2.79b
Make sure blender is in focus and your mouse is over the specific, blender internal text editor window. 

Unlike any other text processing application blender not only needs to be in focus but also requires you to have your mouse over the specific (text editor) subwindow while copying text. 

Just tested with ubuntu 18.04 and blender 2.79 from builder.blender.org